1. Field of the Invention
The present invention relates generally to a multiplexing transmission apparatus and method in a wireless communication system. More particularly, the present invention relates to a multiplexing transmission apparatus and method using modulation and spreading.
2. Description of the Related Art
Generally, wireless communication systems use multiplexing techniques to transmit data. The multiplexing techniques are classified into Time Division Multiplexing (TDM), Code Division Multiplexing (CDM), Frequency Division Multiplexing (FDM), and the like. The multiplexing techniques are one method used for distinguishing services or users. The TDM technique is a method for dividing a specific time interval into several time slots, and transmitting data of a specific user or service over one or more of the divided time slots. The FDM technique is a method for transmitting data of a specific user or service over a specific frequency band among a plurality of predetermined frequency bands. The CDM technique is a method for transmitting data of a specific user or service by spreading the data using one or more codes among a plurality of predetermined codes.
A detailed description will now be made of the CDM technique among the multiplexing techniques. As described above, the CDM technique, that is, the spreading multiplexing technique using codes, is generally used for distinguishing services using spreading codes, or is used as a multi-code technique for allocating a plurality of codes to one service. A modulation technique using such multiplexing techniques uses a method for selecting and transmitting one of spreading codes in an input data information stream.
The CDM method is applied to various systems, particularly to a Code Division Multiple Access (CDMA) mobile communication system and a Satellite Digital Multimedia Broadcasting (S-DMB) system.
The S-DMB system will now be described with reference to the accompanying drawings.
FIG. 1 is a block diagram of a data transmitter according to the S-DMB standard.
Referring to FIG. 1, the data transmitter includes transmission data generators 110, 120 and 130 for generating transmission service data, a control data generator 140 for generating transmission control data, and a CDM multiplexer 150. Among the elements, the transmission data generators 110, 120 and 130 are equal in structure, therefore only one of the transmission data generators will be described, for clarity and conciseness.
If transmission payload data is input to the transmission data generator 110, a Reed-Solomon (RS) encoder 111 performs RS coding on the input data. The RS-coded symbols are input to a first interleaver 112, which is a byte interleaver for interleaving symbols in bytes. The first interleaver 112 interleaves the input data, and outputs the interleaved data to a convolutional encoder 113. Then the convolutional encoder 113 re-encodes the interleaved symbols, and generates coded symbols. The symbols convolutional-coded by the convolutional encoder 113 are input to a second interleaver 114, which is a bit interleaver for interleaving symbols in bits. The output symbols interleaved by the second interleaver 114 are input to the CDM multiplexer 150.
Compared with the transmission data generator 110, the control data generator 140 for generating control data does not include the second interleaver 114. That is, an RS encoder 141, a first interleaver 142 and a convolutional encoder 143 in the control data generator 140 correspond to the RS encoder 111, the first interleaver 112 and the convolutional encoder 113 in the transmission data generator 110. Therefore, in the control data generator 140, the convolutional-coded symbols are input to the CDM multiplexer 150. The symbols output from the control data generator 140 are control data on a pilot channel.
The CDM multiplexer 150 receives the symbols from the transmission data generators 110, 120 and 130, and the control data generator 140, and performs CDM on the received symbols using received pilot symbols. That is, if each of the data generators 110, 120, 130 and 140 is assumed to be one channel, the CDM multiplexer 150 CDM-multiplexes the symbols received from the channels using Walsh codes, and outputs the CDM-multiplexed symbols to a modulator. According to the S-DMB standard, CDM performs multiplexing through orthogonal spreading using 64-length Walsh codes.
Therefore, in the S-DMB, the possible number of channels distinguishable by Walsh codes is 64. However, in the multi-path fading environment, some of the Walsh codes cannot be used for the multiplexing technique due to interference between channels.
FIG. 2 is a functional block diagram of a general Walsh modulator. With reference to FIG. 2, a description will now be made of a structure of the general Walsh modulator.
Referring to FIG. 2, a serial-to-parallel (S/P) converter 210 in the Walsh modulator converts input serial data into parallel data, and outputs the parallel data to a function processor 220. The function processor 220 converts the parallel data into different data depending on an arbitrary one-to-one (bijective) function f( ). If the converted data is denoted by ‘m’, the data ‘m’ is input to a Walsh generator 230, and the Walsh generator 230 generates Walsh codes WN(m) using the input data ‘m’.
A detailed description will now be made of an operation of the Walsh modulator.
The total number of length-N Walsh codes is N. If each of the Walsh codes output from the Walsh generator 230 is denoted by WN(m), m is an element of {0, 1, 2, . . . , N−1}. If n=log2N, an index ‘m’ of a length-N Walsh code is expressed by a length-n bit stream. When there is an arbitrary one-to-one function f( ) in the function processor 220, a relationship between an input n-bit stream and the ‘m’ is defined by f( ). If an inverse function of f( ) is defined as g( ), a receiver selects a Walsh code generated from a transmitter among N Walsh codes, thereby finding a transmitted Walsh code index ‘m’ and detecting an n-bit transmission information stream through a relationship of the g( ).
As described above, in performing CDM transmission using length-64 Walsh codes, the S-DMB transmission technology cannot use some of the 64 Walsh channels due to interference occurring in the wireless channel environment like the multi-path fading environment, causing a waste of channels. In addition, because some of the Walsh channels cannot be used, the total transmission efficiency deteriorates. Such problems occur not only in the S-DMB communication system, but also in the CDM communication system.
Accordingly, there is a need for an improved apparatus and method with increased transmission efficiency in a wireless CDM communication system.